[Multimodal][Core] Optimize multimodal preprocessing cache by hashing image bytes instead of pixel values (#29621)

Signed-off-by: Rahul Steiger <rasteiger@ethz.ch>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

tests/conftest.py

@@ -59,6 +59,7 @@ from vllm.distributed import (
 )
 from vllm.logger import init_logger
 from vllm.logprobs import Logprob
+from vllm.multimodal.base import MediaWithBytes
 from vllm.multimodal.utils import fetch_image
 from vllm.outputs import RequestOutput
 from vllm.sampling_params import BeamSearchParams
@@ -1389,7 +1390,11 @@ class LocalAssetServer:
         return f"{self.base_url}/{name}"
 
     def get_image_asset(self, name: str) -> Image.Image:
-        return fetch_image(self.url_for(name))
+        image = fetch_image(self.url_for(name))
+        # Unwrap MediaWithBytes if present
+        if isinstance(image, MediaWithBytes):
+            image = image.media
+        return image
 
 
 @pytest.fixture(scope="session")